ForeCAT: Using CME Deflections to Constrain their Mass and the Drag

Abstract:

Observations show that CMEs can deflect from a purely radial trajectory yet no consensus exists as to the cause of these deflections. The majority of the deflection motion occurs in the corona at distances where the magnetic energy dominates. Accordingly, many theories attribute the CME deflection to magnetic forces. In Kay et al. (2013) we presented ForeCAT, a model for CME deflections based on the magnetic forces (magnetic tension and magnetic pressure gradients). Kay et al. (2014) introduced an improved three-dimensional version of ForeCAT. Here we study the 2008 December 12 CME which occurred during solar minimum of Solar Cycle 24 (Byrne et al 2010, Gui et al. 2011, Liu et al 2010a,b). This CME erupted from high latitudes, and, despite the weak background magnetic field, deflected to the ecliptic, impacting Earth. From the observations, we are able to constrain all of the ForeCAT input parameters except for the CME mass and the drag coefficient that affects the CME motion. The reduced chi-square best fit to the observations constrains the CME mass range to 3e14 to 7e14 g and the drag coefficient range to 1.9 to 2.4. We explore the effects of a different magnetic background which decreases less rapidly than our standard Potential Field Source Surface (PFSS) model, as type II radio bursts suggest that the PFSS magnetic field decays too rapidly above active regions. For the case of the filament eruption of 2008 December 12 we find that the quiet sun coronal magnetic field should behave similar to the PFSS model. Finally, we present our current work exploring the case of the 2008 April 9 CME.